A mass spectrometer is a device that ionizes molecules or atoms of a sample, then separates the resultant ions according to their mass-to-charge ratio and detects these ions. Various methods for ionizing sample molecules have been developed to date, and electron ionization (EI) is one of the most generally used methods. In electron ionization, sample molecules are introduced into an ionization chamber of a comparatively small capacity, which is placed under a vacuum atmosphere. A filament for generating thermions is provided outside this ionization chamber, and thermions thereby generated are accelerated and injected into the ionization chamber. These thermions come in contact with the sample molecules to ionize these molecules within the ionization chamber. The ions thus produced within the ionization chamber are extracted to the outside by the action of an electric field created by a voltage applied to ion-extracting electrodes (e.g. a lens optical system) provided outside the ionization chamber.
To accelerate the thermions generated by the filament, an electric field with a potential gradient is created between the filament and the ionization chamber by providing a potential difference between them. Due to the action of this electric field, the thermions move toward the ionization chamber and enter the same chamber through an electron injection port provided in the wall of the ionization chamber. In normal modes of electron ionization, the thermions should gain an acceleration energy of approximately 70 eV. Accordingly, the potential difference between the filament and the ionization chamber is set at approximately 70 V.
In an ion source that performs electron ionization, the thermions generated by the filament should be efficiently introduced into the ionization chamber to improve the ionization efficiency within this chamber. For this purpose, the filament in the conventional ion sources is located relatively close to the electron injection port bored in the wall of the ionization chamber. However, if the filament is placed in the vicinity of the electron injection port, the electric field created by the potential difference between the filament and the ionization chamber may penetrate into the ionization chamber and disturb an ion-extracting electric field within the ionization chamber, which is mainly created by the potential difference between the lens optical system and the ionization chamber. Such a disturbance of the electric field will impede the extraction of ions from the ionization chamber.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-373616